Machining holes into turbine blades, in particular into cooled gas turbine blades is an extraordinary difficult tasks, since the airfoils of the blades have convex and concave three dimensional surfaces and the optimal thermal boundary layer during gas turbine operation under high temperature requires a very sophisticated distribution and geometry of the holes, through which cooling gas respectively air is supplied to the gas turbine airfoil's surface. The holes, which can barely be named bore holes, often have bended longitudinal extensions and are of any three dimensional shape. Usually either electrical discharge machining or laser beam drilling is applied to obtain the desired shape of the holes. Both methods do have drawbacks and advantages. Conventional electrical discharge machining is not capable to machine holes into ceramic surfaces. Laser drilling has the disadvantage that the laser beam on the one hand might remove material on locations, where material removal is not desired and on the other hand vaporized material can solidify at other locations of the work piece.
The European patent application EP 0 267 718 A2 suggests to combine both manufacturing methods, wherein during a first step holes are drilled in wax filed hollow gas turbine blade by laser drilling and in a second step a diffuser portion of these holes is machined by electrical discharge machining. Furthermore it is proposed to move the electrical discharge machining device and the laser drill device in the same coordinate system for a higher degree of accuracy. One disadvantage of the apparatus and the method according to the European patent application is the necessary adjustment of both devices relative to each other and the common coordinate system. Often even a new alignment might be necessary for each step of the proposed machining procedure. Since each gas turbine blade has a plurality of holes in a very complex distribution the programming of an apparatus for machining the holes is very complicated, time consuming and can be a source of various mistakes.